The gunman who killed a rabbi and three children at a Jewish school in Toulouse on Monday was wearing a small video camera around his neck and may have filmed the attack, according to the French interior minister.

Claude Guéant said the attacker was wearing "a kind of filming apparatus" on his chest when he struck just before 8am and opened fire on children and adults outside the school. Asked whether the gunman recorded the scene, he said: "We can imagine that."

Guéant said on Tuesday that authorities were combing the internet to see if the killer had posted a video online, but had not yet found any traces.

The biggest manhunt in modern French history is under way to find the motorcycle gunman, who is still at large.

Schools across France held a moment of silence on Tuesday to honour the three children and a rabbi killed in Monday's attack.

The president, Nicolas Sarkozy, visiting a junior high school in Paris, said the children killed "are exactly like you". He said the attack "could have happened here", adding that authorities "will do everything to stop him".

He stressed the need to increase security at synagogues and other Jewish sites in France.

Investigators say the killer is the same person who shot three French soldiers of north African and Caribbean origin in two other attacks in Toulouse and nearby Montauban last week. Three soldiers died and one was injured.

The same heavy, high-calibre weapon was used in all the attacks as well as a powerful Yamaha motorbike used for the getaways which was believed to have been stolen in Toulouse, but which could have been repainted white before the school attack.

The man was wearing a motorcycle helmet during the shootings. He was filmed by CCTV cameras at the school as well as security cameras outside the cashpoint in Montauban last week.

Guéant told French radio: "We're worried about the eventuality that he wants to carry out another attack." He said investigators did not know who the man was, and the inquiry was continuing but had not significantly advanced.

He said "Each hour of work brings new elements. We're checking the past of certain people, we're checking witness accounts, material elements."

He confirmed that police had been sifting through details of former soldiers who might have been expelled from the army for expressing "neo-Nazi opinions" and who "might have a spirit of revenge". But he said that this thread was not particularly placed above other lines of inquiry.

Criminologists in the French media speculated that the killer could be someone with good knowledge and experience of firearms who appeared to have acted without emotion, and in cold blood.

David Cameron has written to the French president, Nicolas Sarkozy, condemning the attack as "senseless acts of brutality and cowardice".

"I was appalled to learn of the recent shootings that France has suffered, including in Toulouse this morning. People across Britain share the shock and grief that is being felt in France, and my thoughts are with the victims, their friends and their families," he wrote.

"I know that France will draw strength and comfort from your resolute leadership at this difficult time. You can count on my every support in confronting these senseless acts of brutality and cowardice."